1. Field of the Invention
The present invention generally relates to speech recognition systems, and more particularly to a method and apparatus for handling out-of-grammar utterances in a spoken conversation.
2. Description of the Related Art
In transactional ASR applications, to carry out speech recognition at a dialog state one often uses a finite state grammar (FSG) as the model of language for the state. However, it is frequently found that grammar designed for a dialog state does not cover everything that users actually say at that state. The present invention addresses the problem of handling the user utterances that are not covered by the grammar (out-of-grammar (OOG) utterances).
For out-of-grammar utterances, the desirable system behavior is to accept, and interpret, utterances that are meaningful for the dialog state, and reject those that are not. When using grammar for recognition, it is very hard to achieve correct acceptance and interpretation because ASR on such sentences using the grammar produces unreliable hypotheses with unreliable confidence scores.
Some known approaches for handling out-of-grammar utterances include use of a background model made from words, phones, or states of the ASR acoustic model. However, these approaches only try to improve rejection of the OOG sentences without regard to sentences that may be meaningful for the state and should not be rejected.